An apparatus adapted to irradiate a laser beam selectively on a sensitive material based on an image recording signal thereby forming an image has been known so far as a film plotter or an image setter. For example, Japanese Patent Unexamined Publication No. 60-203071 discloses a laser plate making apparatus of forming an image by a plurality of laser beams.
On the other hand, along with popularization of computers or development of network techniques typically represented by internets in recent years, a CTP (computer to plate) system of directly making a plate for offset printing from digital image data edited on a computer not by way of a negative film or a positive film has been enabled. Then, the CTP system has attracted attention as a substitute for a PS (pre-sensitized) plate system using a film, which is predominant in offset printing at present.
A system already put to practical use as a plate making system of an offset printing plate used for the CTP system is a print making system using photosensitive materials such as an OPC (organic photo-semiconductor), a silver salt, a hybrid material of a silver salt and a photopolymer and a highly sensitive photopolymer as a blank plate. However, it is necessary for the print making system described above that the blank plate has to be handled in a dark room like that existent PS print systems. Further, the plate making systems described above requires a developing step after the image drawing step to the blank plate like that the existent PS plate system and, therefore, involves a problem for discarding treatment of a liquid developer or the like.
On the contrary, a CTP plate making system using a heat sensitive type blank plate having a response region in an infrared region, the blank plate can be handled in a light room. Further, a great amount of heat energy is charged in an image forming step by a laser beam in this system, thereby an image is formed by thermally converting a portion, to which an image is formed, of a heat sensitive layer from hydrophilic to oleophilic property, so it requires no developing step. Accordingly, such a heat sensitive type CTP system has been noted as a CTP system in the next generation.
Generally, plate making apparatus used for the CTP system are broadly classified, depending on the difference of scanning system, into three types of an outer surface cylinder scanning system, an inner surface cylinder scanning system and a planer scanning system. A laser plate making apparatus of the outer surface scanning system is disclosed, for example, in Japanese Patent Examined Publication No. 51-46138.
As the plate making apparatus used for photosensitive blank plates, a plate making apparatus of an inner surface cylinder scanning system of securing a blank plate to a cylinder inner surface and scanning a laser beam by a rotational end face mirror has been utilized generally, since this can conduct a high speed scanning and also easily cope with different sizes of blank plates. However, the inner surface cylinder scanning type plate making apparatus is not suitable as a plate making apparatus for heat sensitive type blank plates with the reason described below.
That is, since a heat sensitive type blank plate generally has a sensitivity lower by about three digits compared with a photosensitive blank plate, when the inner surface cylinder scanning system is adopted, it requires an expensive solid laser of excellent beam characteristics, for example, an Nd-YAG laser capable of providing an extremely high output energy and having a long focal distance. However, since the sensitive wavelength region of usable blank plates is restricted to 1064 nm as an emitting wavelength of an Nd-YAG laser, the degree of freedom for the design of the blank plate is lowered in the inner surface cylinder scanning type plate making apparatus using the Nd-YAG laser as an image forming laser.
On the contrary, a semiconductor laser having a central light emission wavelength region near 750-880 nm is inexpensive compared with the Nd-YAG laser. Accordingly, use of the semiconductor layer for the image forming laser is preferred in order to reduce the apparatus cost of the heat sensitive type CTP system. However, since no long focal distance can be available in the semiconductor laser in view of beam characteristics, it is difficult to adopt the inner surface cylinder scanning system in the plate making apparatus using the semiconductor laser.
Accordingly, in the plate making apparatus using the semiconductor laser, an outer surface cylinder scanning system, that is, a system of winding a blank plate around the outer surface of a cylinder, and irradiating a laser beam to the blank plate from an optical head disposed near the cylinder outer surface is adopted. The plate making apparatus of this type is adapted, for example, such that a laser beam irradiated from a semiconductor laser is transmitted through an optical fiber and introduced to the optical system of an optical head disposed near the cylinder outer surface and a laser beam focused by an objective lens at the top end of the optical system to the blank plate at the cylinder outer surface.
In the plate making apparatus of the outer surface cylinder scanning system described above, with an aim of increasing the plate making speed, an image is formed by a so-called multi-channel system of using a plurality of semiconductor lasers to increase the number of scanning lines per one rotation of the cylinder.
Then, in a general multi channel system plate making apparatus, a plurality of laser beams are arranged each at an equal interval in line and the beams are formed into a group of beams parallel with each other and the beam group is introduced to a set of optical systems.
However, when an image is formed by a plurality of infrared laser beams arranged in line, heat of infrared rays is absorbed in the heat sensitive layer, as well as a great amount of heat is also generated by chemical reaction in the heat sensitive layer. Then, the heat diffuses to the periphery by heat conduction while elevating the temperature of the blank plate. Accordingly, the temperature of the image area of the blank plate formed with an image by a beam situated at the center of the line is higher compared with that in the image region of the blank plate formed with an image by a beam situated on the end of the line.
As described above, when an image is formed by a plurality of infrared laser beams disposed in line to the heat sensitive blank plate to be formed with images by thermal reaction, since a temperature distribution is caused to the blank plate upon image formation, it is difficult to form an image uniformly over the entire image formation region. That is, there is a room for the improvement of the image quality of the printing plate obtained by this method.
On the other hand, in the process color printing, a color image is separated into that of four colors, namely, Y (yellow), M (magenta), C (cyan) and K (black), and a plate for each color is made, and each of images is printed with an ink of a corresponding color by using the four plates. Then, color printed matters of good quality can be obtained by overlapping images printed by the four plates with inks of different colors on an exact position of paper. Positional alignment for each of the plates in a printing machine is carried out by disposing one side as a reference to each of the plates and aligning the sides to each other. Accordingly, also in the plate making, an image has to be formed at an accurate position with the side being as a reference.
However, the plate making apparatus of the outer surface cylinder scanning system in the prior art still has a room for the improvement in view of convenient and accurate positioning upon attaching the blank plate to the cylinder.
Further, Japanese Patent Unexamined Publication No. 7-1849 discloses a material for forming a heat sensitive layer constituting a heat sensitive type blank plate, which contains microcapsules containing an oleophilic ingredient in the inside and destroyed by heat, hydrophilic binder polymer having functional groups capable of three dimensional cross linking and functional groups capable of reacting with the oleophilic ingredient, and photoreaction initiator for initiating three dimensional cross linking reaction of the hydrophilic binder polymer. However, the printing plate made by the existent method using the heat sensitive blank plate having the material as a heat sensitive layer is insufficient in the printing resistance for an image area and still leaves a room for improvement in the printing quality of the obtained printing plate.
The present invention has been accomplished taking notice on the problems in the prior art described above, and it is a subject thereof to remarkably improve the quality of the image to be formed and the printing quality in the image area, upon making the heat sensitive blank plate into a printing plate by the outer surface cylinder scanning system plate making apparatus and, further, enable to accurately position images of four colors by a convenient method in a short period of time upon process color printing.